In the final moments of merging, two neutron stars don't merely emit gravitational waves, but a catastrophic explosion that echoes across the electromagnetic spectrum. The arrival time difference between light and gravitational waves enables us to learn a lot about the Universe. Image credit: University of Warwick / Mark Garlick.

“Dark matter is interesting. Basically, the Universe is heavier than it should be. There’s whole swathes of stuff we can’t account for.” -Talulah Riley

One of the most puzzling facts about the Universe is that 95% of the energy in it, in the forms of dark matter and dark energy, are completely invisible, and have never been directly detected. Perhaps, the story goes, it’s our theory of gravity that’s to blame, rather than needing new components in the Universe. While dark matter and dark energy can explain a whole slew of observations, gravity modifications do a better job of explaining galactic rotation, but require altering Einstein’s theory of gravity.

The cosmic web is driven by dark matter, with the largest-scale structure set by the expansion rate and dark energy. The small structures along the filaments form by the collapse of normal, electromagnetically-interacting matter. Image credit: Ralf Kaehler, Oliver Hahn and Tom Abel (KIPAC).

But merging neutron stars provide a unique test: electromagnetic and gravitational waves both originate from an ultra-distant source over 100 million light years away. The first signals arrive separated by mere seconds, allowing us to constrain models where gravity and light are bent (and delayed) differently by the presence of masses. While theories like Bekenstein’s TeVeS and Moffat’s Scalar-Tensor-Vector predict differing delays by years, the observed arrival time difference was just 1.7s.